Making three vertices symmetric without moving middle vertex
You can use snap to symmetry feature.
Select vertexes that you want to move in mirrored position. Then in menu choose Mesh -&gt; Snap to Symmetry.
Then you need to adjust threshold using Adjust last operation (F9 or panel in bottom right corner)
Optionally, you can edit factor, to define what vertexes you want to move.

Then you can use X-mirror option to keep this vertexes in sync. This setting is located in 3 places: